Polystyrene-sulfonate (PSS) is an ionically charged polymer classified as a polyelectrolyte or ionomer depending on the concentration of the sulfonate ions present. It has been used in a number of applications. For example, PSS has been used in osmosis membranes for water purification, cation exchange resins for treatment of hyperkalemia, and electrolyte layers in battery applications and electronic devices. PSS also has been used as an additive to aid in bitumen strength or oil remediation. Its ubiquitous use is largely due to the fact that PSS represents one of the few commercially available options for polyelectrolytes due, at least in part, to the fact that its parent precursor, polystyrene (PS), is produced in large quantities. PSS, however, often is considered too brittle due to its glassy state and Tg.
Copolymers can present opportunities to design materials with different and/or potentially advantageous properties when compared to homopolymers of their individual constituents. Although polyethylene and polystyrene are ubiquitous plastics, the synthesis of ethylene-styrene (ES) copolymers has been challenging due, at least in part, to the markedly different monomer reactivities, and, as a result, has been deemed non-viable in view of the limitations of traditional Ziegler-Natta catalyst systems. Ethylene and styrene monomers can be copolymerized to form ethylene-styrene copolymers, however, the catalysts used are complex, styrene incorporation is not precise, and/or it can be difficult to achieve high styrene content due at least in part to the differences in reactivity between ethylene and styrene.
The advent of well-defined “single-site” or molecular catalysts over the last few decades has brought new attention to the efficacy and utility of ES copolymers. A broad scope of material properties are realized when traversing from low to high styrene (S) content which can translate to a variety of applications for compatibilizers, packaging, films, foams, automobiles, construction materials, and/or bitumen modifiers. However, producing ES copolymers with high S content (>70% w/w) remains a formidable challenge, due to the fact that high S feed ratios typically are required and/or the processes can create difficult issues regarding homopolymer formation, product irreproducibility, and/or the discrete/complex nature of the molecular catalysts. Generally, it is known that the copolymerization of two different monomers can create a copolymer having a statistical distribution of the two repeat units, rather than a precise distribution.
An alternative strategy to ethylene copolymers is ring-opening metathesis polymerization (ROMP) of strained monocyclic alkenes or acyclic diene metathesis (ADMET) polymerization of linear dienes followed by hydrogenation of the backbone olefins. In both cases a singular monomer can be used to impart periodic chain branching with functionalities that are analogous to copolymer systems. Precision polymers are a subset of these materials that incorporate branches at exactly spaced periodicity along a polyethylene chain. The region-specific branching of these systems can lead to well-defined properties for specialty applications.
Precision ES copolymers, however, have been scarcely reported. A product has been reported having a phenyl substituent on every 19th backbone carbon through ADMET polymerization (Watson, M. D. et al., Macromolecules 2000, 33, 8963). This material, equivalent to 30% w/w S, was semi-crystalline and thermal analysis revealed complex melting transitions between −22.5 and −1.5° C. Another product has been reported that resulted from a highly regioregular (99.9% head-tail insertion) polymerization of cis-3-phenylcyclooctene; resulting in a precision ES copolymer with phenyl branches at every 8th carbon (55% w/w S) following hydrogenation (Kobayashi, S. et al., J. Am. Chem. Soc. 2011, 133, 5794). This material was amorphous and had a glass transition temperature (Tg) of −15° C.
There remains a need for sulfonated copolymers that are amorphous, have a relatively low glass transition temperature, have reduced phenyl branch periodicity, and/or overcome one or more of the foregoing disadvantages regarding copolymers, such as PSS and highly-sulfonated PSS.